There has been an abruptly growing demand on information transmission in association with the recent widespread use of personal computers and the Internet, so optical transmission with a high transmission rate has become widespread. An optical waveguide has been used as an optical interconnection in such optical transmission.
By the way, an optical branching circuit and an optical multiplexing circuit serving as basic elements are indispensable to an integrated optical circuit, and an optical waveguide branched to provide a Y shape has been conventionally known. As shown in FIG. 1, a conventional Y branch optical waveguide is structured by connecting a main waveguide 1, a taper waveguide 2, and branching waveguides 3 and 4, and a branch point 5 is present between the taper waveguide 2 and the branching waveguides 3 and 4.
A reduction in light loss is an important breakthrough in such Y branch optical waveguide, and one possible method for achieving the breakthrough is to increase the radius of curvature of each of the branching waveguides 3 and 4 each curving in the form of an arc. In this case, however, the size of a circuit must be increased. In actuality, the size of the circuit is restricted by the size of a substrate, so an increase in radius of curvature is limited.
In addition, the branch point 5 must be sharp for a reduction in light loss. However, the point cannot be of a completely sharp structure owing to, for example, the accuracy of patterning or etching. An optical central portion having the highest light intensity (the central axis of an optical propagation mode) is scattered at the branch point 5, thereby causing a large branch loss.
Furthermore, when implementation is performed by connecting incident and output fibers to a 1×N optical splitter, an offset may occur between the incident fiber and the incident straight line-optical waveguide of an optical circuit owing to, for example, the tool accuracy of a jig and the mechanical accuracy of an alignment device. In this case, a higher-order mode and a radiation mode are excited in addition to a basic mode in an optical waveguide, so a variation in branching ratio occurs.
To cope with the above problem, there has been proposed that, in a branching multiplexing optical waveguide circuit obtained by: connecting a taper waveguide to a main waveguide; connecting multiple branching optical waveguides each having an inflectural point to a branch point of the taper waveguide; and connecting an output waveguide to each of the branching optical waveguides, an offset is arranged at the connecting point of the point of inflection of each branching optical waveguide and the corresponding output waveguide, and a gap is arranged between two arbitrary branching waveguides at the branch point of the taper waveguide (see, for example, Claims in Patent Document 1 below). In such incident light waveguide, when the central axis (central axis of an optical propagation mode) h of the intensity distribution (field distribution) of propagating light and the geometrical central axis a of an incident light waveguide (core portion) 7 coincide with each other, and the intensity distribution of light is of a shape symmetric with respect to the central axis h of the intensity distribution of light as shown in FIG. 2, a light branching optical waveguide having a reduced branch loss and a reduced variation in branching ratio can be obtained by means of the above method.
However, for example, when the optical waveguide has a curve structure, the case where the central axis (central axis of an optical propagation mode) h of the intensity distribution of light and the geometrical central axis a of the incident light waveguide (core portion) 7 do not coincide with each other as shown in FIG. 3, or the case where the intensity distribution of light is of a shape asymmetric with respect to the central axis h of the intensity distribution of light as shown in FIG. 4 even though the central axis h of the intensity distribution of light and the geometrical central axis a of the incident light waveguide (core portion) 7 coincide with each other occurs. In each of those cases, there arises a problem in that the branching ratio of the light branching optical waveguide cannot be equal even when such offset structure as described above is arranged.
On the other hand, when light propagating in an incident light waveguide has an intensity distribution (field distribution) of light asymmetric with respect to the geometrical central axis of the incident light waveguide, making the shape of the distribution symmetric requires a long straight line portion, so there arises a problem in that the size of a module increases.
A multi-mode interference (which may hereinafter be abbreviated as “MMI”) type Y branch optical waveguide has been known other than the above light branching optical waveguide using a taper waveguide, and various kinds of such MMI waveguides have been proposed (see, for example, Claims in Patent Document 2 below). The MMI type Y branch optical waveguide is composed of an incident waveguide, a multi-mode waveguide portion, and two output waveguides. When light of a basic mode propagating in the incident waveguide enters on the central axis of the multi-mode waveguide, light of the basic mode (n=0) and light of a higher-order mode (n=2) are excited, and the waveform of propagating light deforms owing to interference due to a difference in phase speed between light of the basic mode and light of the higher-order mode. The propagating light has an intensity distribution having two peaks at a site where light of the basic mode and light of the higher-order mode are different from each other in phase by π. Arranging the two output waveguides in correspondence with the peaks can achieve the branching of light to a branching ratio of 1:1 (equal) (see paragraphs [0038] and [0039] in Patent Document 2 below). Therefore, light can be branched at a short distance as compared to a taper waveguide. In addition, there does not arise a problem in that such optical central portion having the highest light intensity as described above is scattered at a branch point, to thereby cause a large branch loss.
However, the above branching ratio of 1:1 (equal) in branching of light is achieved only in the case where the mode of light propagating in the incidence waveguide is a basic mode alone, the basic mode is symmetric with respect to the central axis of the incidence waveguide, the central axis of the incidence waveguide and that of the multi-mode waveguide coincide with each other, and the multi-mode waveguide is of a shape symmetric with respect to its central axis. That is, for example, in the case where the intensity distribution (field distribution) of light propagating in an optical waveguide on an incident side is asymmetric with respect to the geometrical central axis of the optical waveguide, there arises a problem in that the branching ratio of light cannot be equal even in an MMI type light branching optical waveguide.
In actuality, an incident light into a multi-mode optical waveguide often includes components of a higher-order mode and a radiation mode in addition to a basic mode propagating in an incident light waveguide. In addition, for example, when the incident light waveguide has a curvature, the basic mode is generally asymmetric.
Furthermore, in an MMI type light branching optical waveguide, the position at which light of a basic mode and light of a higher-order mode interfere with each other varies depending on a wavelength. Therefore, there arises a problem in that each of a loss of light intensity and a branching ratio is dependent on the wavelength. That is, the design of the MMI type light branching optical waveguide must be changed in accordance with the wavelength of light, so there arises a problem such as a reduction in efficiency of production.    Patent Document 1: JP 04-213407 A    Patent Document 2: JP 2000-121857 A